It is known to produce I and Q channel signals in various ways. For example, a known method uses a transmission line quadrature hybrid incorporating wound components which make it unsuitable for fabrication by integrated circuit technology. Alternatively, at ultra high frequencies (UHF), striplines may be used. Such hybrids suffer from the disadvantage in that they are, essentially, narrow band devices. A second known method uses a digital divider. However, as the divider employs digital techniques, it cannot handle quadrature phase shift at low levels or where the signal amplitudes vary to any great extent.
A further known method is that utilising a resistance-capacitance network to produce the required phase shift. Such a network is shown diagrammatically in FIG. 1 of the accompanying drawings. The network comprises two arms 10, a first arm containing a resistor R.sub.1 and a capacitor C.sub.1, and a second arm containing a resistor R2 and a capacitor C.sub.2. Providing R.sub.1.C.sub.1 is equal to C.sub.2.R.sub.2, a signal applied at V.sub.APP will be split into two signal channels in quadrature phase, a first signal channel being advanced in phase by 45.degree. and the second signal channel being retarded in phase by 45.degree.. The values of the resistors R.sub.1, R.sub.2 and of the capacitors C.sub.1, C.sub.2 are chosen for the specific frequency of the applied signal. Although of low cost and offering an acceptable tolerance sensitivity (of the designed frequency), variations in the frequency of the applied signal cause relatively high losses in the network, and hence, it is essentially a narrow bandwidth device if the network loss is not to exceed 3 dB.
In general, quadrature signal generators have an accuracy tolerance which depends on the tolerance of the components forming the circuit. In integrated circuits, because of their small size, high accuracy of component values is extremely difficult to achieve.